linux-kernel - [PATCH 3.16 109/134] metag/uaccess: Check access_ok in strncpy_from

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Message-ID: <lsq.1503062000.389457669@decadent.org.uk>
Date:   Fri, 18 Aug 2017 14:13:20 +0100
From:   Ben Hutchings <ben@...adent.org.uk>
To:     linux-kernel@...r.kernel.org, stable@...r.kernel.org
CC:     akpm@...ux-foundation.org, "James Hogan" <james.hogan@...tec.com>,
        "Al Viro" <viro@...iv.linux.org.uk>, linux-metag@...r.kernel.org
Subject: [PATCH 3.16 109/134] metag/uaccess: Check access_ok in
 strncpy_from_user

3.16.47-rc1 review patch.  If anyone has any objections, please let me know.

------------------

From: James Hogan <james.hogan@...tec.com>

commit 3a158a62da0673db918b53ac1440845a5b64fd90 upstream.

The metag implementation of strncpy_from_user() doesn't validate the src
pointer, which could allow reading of arbitrary kernel memory. Add a
short access_ok() check to prevent that.

Its still possible for it to read across the user/kernel boundary, but
it will invariably reach a NUL character after only 9 bytes, leaking
only a static kernel address being loaded into D0Re0 at the beginning of
__start, which is acceptable for the immediate fix.

Reported-by: Al Viro <viro@...iv.linux.org.uk>
Signed-off-by: James Hogan <james.hogan@...tec.com>
Cc: linux-metag@...r.kernel.org
Signed-off-by: Ben Hutchings <ben@...adent.org.uk>
---
 arch/metag/include/asm/uaccess.h | 9 +++++++--
 1 file changed, 7 insertions(+), 2 deletions(-)

--- a/arch/metag/include/asm/uaccess.h
+++ b/arch/metag/include/asm/uaccess.h
@@ -189,8 +189,13 @@ do {
 extern long __must_check __strncpy_from_user(char *dst, const char __user *src,
 					     long count);
 
-#define strncpy_from_user(dst, src, count) __strncpy_from_user(dst, src, count)
-
+static inline long
+strncpy_from_user(char *dst, const char __user *src, long count)
+{
+	if (!access_ok(VERIFY_READ, src, 1))
+		return -EFAULT;
+	return __strncpy_from_user(dst, src, count);
+}
 /*
  * Return the size of a string (including the ending 0)
  *